Semi-catalytic infra-red heat producing unit



Oct. 22, 1968 1.. c. HARDISON 3,407,025

5EMI-CATALYTIC INFRA-RED HEAT PRODUCING UNIT Filed Oct. 19, 1964 Figure/V VEN TOR Leslie 0. Hard/son A [TOR/V15 Y8 United States Patent3,407,025 SEMI-CATALYTIC INFRA-RED HEAT PRODUCING UNIT Leslie C.Hardison, Chippewa Falls, Wis., assignor to Universal Oil ProductsCompany, Des Plaines, III., a corporation of Delaware Filed Oct. 19,1964, Ser. No. 404,831 Claims. (Cl. 431'329)' ABSTRACT OF THE DISCLOSUREA semi-catalytic infra-red heater operating on gaseous fuel comprising afirst stage non-catalytic alloy metal screen and a second stage catalystcoated alloy metal screen spaced a short distance downstream therefrom,said screens being supported by. a housinghaving an air and fuel supplyinlet thereto. The screeens may be flat and parallel, or tubular andconcentric.

The present invention is directed to a semi-catalytic infra-red heatproducing unit and more particularly to the design and construction ofan all-metal infra-red heater unit providing both non-catalytic andcatalytic combustion of a fuel-air mixture as the latter passes throughtwo separate stages of special metal gauzes.

There have been many types of infra-red heat producing units devised toeffect either a non-catalytic or a catalytic burning of fuel-airmixtures over or through a radiant surface. The earlier catalytic formsof infrared heaters utilized activating agents impregnated onto asbestosfiber gauze to assist in thecatalytic oxidation of a fuel-air mixture;however, such materials were fragile and limited to low temperatureoperations. lngeneral, temperatures above about 900 F. would lead tocharring and breakage. Asa result, porous ceramics or metal alloy mesheshave been preferred for non-catalytic and catalytic heat generatingunits.

and a resulting white hot ceramic. However, since the source temperatureis high, there is no possibility. of easily controlling the actual wavelength of the radiant energy reaching the material to be heated.

In connection with the use of electrically energized heaters, there arevarious disadvantages in that the surface of the heater itself must beat a relatively high temperature in order to produce radiant energy. Asa result, it is sometimes difficult to provide the optimum wave lengthfor thework being treated. Also, electrically energized heaters are notreadily adaptable to proportional control since such heaters are eitherOn or Off and it is difficult to secure intermediate ranges 'of radiantenergy. Furthermore, the reflector means utilized in con nection withenergized infra-red heaters gradually become covered with dirt depositsso as to reduce their efficiency, i.e., as less energy is reflectedthereis a subsequent heat loss. i

Various of the infra-red heat producing units incorporatingcatalytically coated alloy gauzes have beenof particular advantage inproviding controlled burning of fuel-air mixture in a flameless manner.Actually, opera- 'ice tions may provide temperature emissions over awide range of temperature of from about 900 F. to about 1800 F. However,in studying gas emissions from the various types of catalyticallycoated, as Well as from uncoated gauze units, there has been found thatthere is relatively high emissions of carbon monoxide. On the otherhand, it 'has been found that by combining .noncatalytic metal screenswith catalytically coated alloy screens in a particular manner, and inaccordnce with the present invention, there can be effected a greaterefficiency and more complete combustion of the fuel-air stream, while atthe same time obtaining the desired benefits of controllability oftemperature and longer wave length infra-red radiant energy leaving theunit.

It is thus a principal object of the present invention to provide aninfra-red heat producing unit which utilizes an interior or upstreamnon-catalytic all-metal gauze surface means to effect initialdistribution and burner surface in combination with spaced exterior ordownstream catalytically coated all-metal alloy gauze means that caneffectively complete the oxidation of unburned gaseous componentsleaving the first surface.

It is also an object of the present invention to provide an all-metalunit that effects a first stage non-catalytic oxidation of the fuel-airmixture and a second stage catalytic oxidation that is carried out onthe catalytically coated gauze surface spaced a short distance away fromand downstream of the first stage layer.

It is still another object of the present invention to use an all-metalreflector screen or gauze that is adjacent to and in combination withthe first stage non-catalytic infra-red burner screen so as to eliminatethe use of mineral wool or other non-metallic diffuser materials.

It was found that certain of the earlier designed units had a problem ofback-flashing of the fuel-air mixture into the manifold section orplenum of the housing, particularly after a gradual heat build-up in thediffusion packing or in connection with distributor screens that werespaced upstream from the infra-red heat emitting surface. Apparently,small -cinder-like particles within a mineral wool distribution layerprovided hot spots which in turn could initiate back-flashing into thefuel inlet zone. Also, in connection with the use of one or more metalalloy screens which were used as reflector or distributing members andwere in spaced positions back of or up stream from the infra-red heatemitting screen, would serve to provide a stepwise back-flashing of thefuel-air mixture into the inlet zone. Inother words, it appears thatthere was a stepwise temperature build up and heat absorption by thespaced upstream screens, particularly at low flow rates, such that therewas sufficient heat carried back to cause combustion in the inlet zone.As a result, it is found that preferred designs utilize one or more heatreflector screens directly adjacent the first stage of fuel-airoxidation and infra-red heat generation.

In one embodiment, the present invention provides a catalytic radiantheating apparatus which comprises in combination, an internal gaspervious first stage combustion element of at least two adjacentuncoated metal alloy gauze members adaptedtto have a gaseous fuel streampass therethrough, whereby at least one downstream member serves as aburner screen and at least one upstream member serves as a radiant heatshield on the fuel inlet side, a second stage combustion element of atleast one gas pervious metal alloy gauze having a noble metal gasoxidizing catalytic surface that is spaced a short distance downstreamfrom the face of the burner screen of said first stage element, andconfined housing means having air and fuel supply inlet means theretoenclosing the upstream face of said radiant heat shield member of saidfirst stage element and in addition encompassing the open spacingbetween said first and second stage combustion elements.

In another embodiment, the present invention provides an infra-red heatproducing unit adapted to provide high temperature radiant heating above900 F., and comprises in combination, a metal alloy gas combustionscreen directly adjacent to and coextensive with at least one all-metalalloy gas distributing-reflective screen, a housing means having air andfuel supply inlet means thereto encompassing the upstream side of saidadjacent -screens, at least one alloy metal catalytic screen having anoble metal gas oxidizing surface that is spaced a short distancedownstream from the outer surface of said gas combustion screen andproviding an unobstructed free area over the surface of said combustionscreen, whereby there may be the first stage non-catalytic oxidation ofgases on said combustion screen and a second stage catalytic combustionon said catalytic screen of the gases leaving the former.

A preferred design utilizes fine mesh screen in each of the stages offuel-air oxidation, as well as for the gasdistributing gas-reflectivescreen, with such screen being in the 20-70 mesh range having wire sizesfrom about 0.005 inch to about 0.02 inch diameter. In all cases, thealloy utilized shall be of chrome-nickel alloys, i.e., Nichrome, Chromeland the like, capable of withstanding high temperature ranges of theorder of about 1600 F.

or more.

A preferred burner unit also has provision for effecting a lighting ofthe fuel-air mixture at the surface of the interior non-catalytic heatproducing screen. For example, a suitable small opening or light-offhole may be provided through the outer catalytically coated gauze orouter portion of the housing.

Additional constructional features and advantages of operation may benoted upon reference to the accompanying drawing and the followingdescription thereof.

FIGURE 1 of the drawing is a sectional elevational view through oneembodiment of an improved two-stage semi-catalytic infra-red heatproducing unit.

FIGURE 2 of the drawing is a sectional elevational view of a cylindricalform of two-stage, semi-catalytic infra-red heat producing unit.

Referring now to FIGURE 1 of the drawing, there is indicated a housing 1having a fuel-air inlet means 2 which leads to an interior plenumchamber or fuel distributing zone 3. Extending interiorly across thehousing 1, as well as over the plenum zone 3, is a relatively large meshsupporting screen 4, an uncoated fine mesh reflector or radiation screen6 and an uncoated burner screen 5. In the present embodiment the edgesof the screen traversing the housing are held by a spacer member 7clamped against an outwardly projecting flange 8 by a cover ring 13 andsuitable bolting means 9. A pair of insulating gasket members 10, whichmay be of asbestos, Fiberfrax or other suitable temperature resistantinsulating material, are used each side of the screen members 4, and 6to partially preclude the transfer of heat from the latter to the flame8 and housing 1.

The present embodiment shows the spacer member 7 as being of a channelshape and extending circumferentially around the edge of the housingabove the upper burner screen 5 so as to provide an intermediateunobstructed spacing between the interior burner screen 5 and anexternal catalytically activated burner screen 11. Where desired, asuitable heavier gauge wide mesh guard screen 12 is superimposedexternally over the catalytic screen 11 and held in place along with thelatter by the "cover ring 13. The spaced bolting means 9 also serve toclamp the cover ring 13 against the edges of the outer screens 11 and 12as well as against the outer flange portion of spacer member 7 toprovide a completed burner unit.

In accordance with the present improved infra-red burner design andconstruction, the catalytically coated outer screen 11 is provided offine mesh chrome-nickel alloy, as noted hereinbefore in the 20-70 meshrange, but more preferably of about 40 mesh with 0.010 inch diameterwire, such that there may be uniform distribution of the partiallyoxidized fuel-air mixture leaving the inner burner screen 5 over theentire cross sectional area of burner unit. The catalytic coating maycomprise platinum, palladium or a combination thereof either alone or incombination with one or more of the other members of the platinum groupof metals such as ruthenium, iridium, rhodium, etc. Also somepercentages of other activating components, such as thorium, tungsten,cesium and the like may be applied in combination with the one or moreplatinum group metals. Deposition of the coating may be carried out inthe manner similar to that set forth in the Suter et al. U.S. Patent No.2,720,494, issued Oct. 11, 1955, where alloy metal wire or ribbon iscatalytically activated to provide a desirable form of gas oxidizing orincinerating element. It is believed unnecessary to provide detailherein as to the means for applying and activating the coating inconnection with the infrared heat generating member of this invention,since reference may be made to such patent for preparationand coatingmethods.

In the operation of the unit of FIGURE 1, the fuelair mixture enters theplenum chamber 3 by way of inlet 2 and is distributed from the latter byway of support screen 4 and reflector screen 6 to the burner screen 5.By providing flame or spark ignition of the fuel-air mixture on theouter surface of the burner screen 5 there will be flame and heating ofthe screen 5 to in turn provide infra-red radiant heat therefrom.Initially, there may be a small amount of visible flame extending overthe surface of burner screen 5; however, generally under hightemperature radiant conditions, when screen 5 has become heated to abovea cherry red temperature, there will be little or no visibility to theflame carrying outwardly from the surface of the screen. The hotcombustion gases leaving the surface of burner screen 5 is passedthrough the internal open space being provided by the height of channelspacer member 7 so as to contact the surface of the catalytically coatedburner screen 11. The latter will effect the catalytic oxidation of theunburned fuel-air components as well as oxidation of carbon monoxidebeing entrained with the gaseous product stream leaving the first stageburner screen 5. In other words, the activated or catalytically coatedburner screen 11 will serve to act as a second stage of oxidation or aclean-up" stage for the gaseous mixture leaving the burner unit. After ashort period of operation, the outer screen 11 will provide a glowinginfra-red heat generating surface to emit heat along with the innerscreen 5 so that the total infra-red heat generation from both burnerstages is effective to radiate outwardly to a work material that is toreceive heat from the burner unit.

It is not intended to limit the construction to any one means foreffecting the lighting of the fuel-air mixture on-the surface of thefirst stage burner screen 5; however, for illustrative purposes, FIGURE1 of the drawing indicates that a lighting hole 14 may be providedthrough a small portion of the-outer catalytically coated screen 11 aswell as through guard screen 12, where such screen is being used as anouter protection over the face of the unit. In an alternativearrangement, a lighting hole such as 15 may be provided through the wallof the channel shaped spacer member 7 as well as through an opposingportion at 15 in the cover ring 13.

It may be noted that the present embodiment indicates a screen 11 toprovide the second stage of combustion for the infra-red heat producingunit, although in some instances, it may be desirable to provide aplurality of catalytically coated screens over the outer surface of theheat producing unit such that there is adequate complete combustion ofall of the gases leaving the unit and substantial elimination of any COfumes therefrom. It should also be noted that it is a particular featureof the present two-stage construction to utilize a definite spaceddistance as provided by spacer member 7 which will maintain anunobstructed spacing on the downstream face of the burner screen 5. Thisspace provides for the redistribution of unburned gases as well ascombustion gases to the surface of the catalytically coated screenmember 11. The actual amount of spacing may, for example, be from about/8 inch to about 1 inch, but will vary in accordance with the size ofthe burner unit, gas flow rates and mesh sizes, which may be utilizedfor both the first and second stage burner screens.

It is a still further feature of the improved construction to utilize atleast two adjacent screens at the first stage of fuel-air oxidation. Inother words, by having two or more uncoated screens at this zone, it ispossible to have at least one of the internal screen members to serve asa distributor or radiation screen causing heat generated on the outersurface of screen 5 to be reflected outwardly from the heater unit. Inany case, there should not be any spacing between the burner screenmember, such as 5, and an upstream reflector or radiant member, such as6, that would permit a stage-wise absorption of heat backwardly into theplenum or fuel distribution zone, which would in turn lead to theproblems of back-flashing described hereinbefore.

Referring now more particularly to FIGURE 2 of the drawing, there isindicated a cylindrical or tubular form of infra-red heat producing unithaving a circularly shaped end closure member 16 having a central fuelinlet opening 17. Opposing the latter and projecting inwardly from thecircular portion of the closure member 16 is a hub portion 18 adapted tosupport cylindrically formed screen members 19 and 20. The latter areuncoated alloy screen members which may be in the 20 to 70 mesh range,but preferably of the order of about 40 to 50 mesh so as to provide aninitial stage of burning and resulting heat radiation for a fuel-airmixture introduced by way of the inlet 17. The downstream end of themembers 19 and 20 are supported on a suitable shoulder portion 21 of acircular support member 22 which in turn is held internally within acircular end closure member 23. The latter has a small flange section 24which is of a diameter approximately equivalent to the external diameterof the closure member 16 at the inlet end of the unit whereby there maybe end supporting surfaces for an outer second stage catalyticallycoated oxidizing screen 25. The outer oxidizing screen 25 is coveredwith a suitable metallic deposition or coating of an oxidation catalyst,preferably a platinum group metal, such as described hereinbefore inconnection with the teachings of the Suter et a1. Patent No. 2,720,494.For assembly purposes, screen 25 may be tack-welded or otherwiseattached to the outer periphery of closure member 16 and to, flange 25on closure 23. In addition, for lighting purposes, there may be asuitable lighting hole 26 through a small portion of the surface of theouter catalytic screen 25.

The operation of the embodiment of FIGURE 2 is similar to that describedin connection with FIGURE 1 except that radiant heat is emitted radiallyoutwardly from a tubular unit which may be cylindrical or of polygonshape. A fuel-air mixture entering inlet 17 carries to an inner plenumzone 27 within the interior of screen 19, which serves as a heatradiation shield, as well as distributor member, such that following alight-off through hole 26 there is a burning of fuel over the entireouter surface of burner screen 20. Following the initial oxidationstage, the entire outer surface of the latter screen becomes glowing andhas an almost invisible high temperature flame that is capable ofproducing high temperature infra-red heat therefrom. The color of theburner screen 20 and the temperature therefrom will, of course, dependupon the rate of gas flow through the unit and the rate of oxidationacross the surface of the screen 20. At the second stage of combustion,being provided by the outer catalytically activated screen 25, therewill be a substantial completion of the oxidation of unburned portionsof the fuel-air mixture as well as oxidation of carbon monoxide tocarbon dioxide that is entrained with the combustion products. Hereagain, after a period of initial operation, the outer catalyticallycoated screen 25 will provide a high temperature infra-red heatproducing glow which will carry outwardly to material that is to beheated.

It should also be noted in connection with the embodiment of FIGURE 2that space provided between the inner burner screen 20 and the secondstage burner screen 25 so as to permit adequate oxidation to take placeacross the outer surface of the first stage burner screen 20 withouthinderance from an adjacent outer screen. In addition, there is suitablespace for the redistribution of combustion gases and unburned. fuel touniformly pass to the surface of the outer second stage screen 25 for acompletion of oxidation of unburned components in the gas flow.Generally, a single outer activated screen such as 25 will suffice toeffect the second stage of burning; however, in some instances of highflow rates, it may be desirable to have more than one coated andcircumscribing screen. Also, it may be'necessary to provide a suitableguard screen around the catalytically coated screen to preclude thelatter from being deformed orscraped during installation procedures.

Further, it may be noted that at least two uncoated screens, 19 and 20,are used adjacent one another at the first stage of combustion wherebythere is at least one inner distributor and reflector screen to assistin precluding excessive backward radiation of heat into the plenum orfuel inlet portions of the unit.

Small cylindrical heater units may be used singly or in groups toprovide specialized heating operations. Optional arrangements of units,such as rows, multiple rows, and other geometric patterns thereof, maybe used with or without reflector means to provide for the directing ofinfra-red heat to a particular heat absorbing area. Also, fuel gas andair may be supplied from header means or from individual supply pipesand air aspirator means, since it is not intended to limit the presentinvention to any one form of fuel distribution system.

In connection with a series of test operations on an improved infra-redheat generating panel having the spaced combination of non-catalytic andcatalytic surfaces, it was found that readings showing the hydrocarboncontent in the effluent gases directly above an open lightoif holeaveraged several points higher than readings above portions of the outercatalytically activated screen. In other words, the tests showed asignificantly greater advantage for the use of the second stagecatalytic oxidation in effecting the completion of fuel burning.

Specifically, in a series of test operations, a unit was used which wassimilar to FIGURE 1 having an internal uncoated 30 mesh screen with 0.01inch diameter wire of Chromel C as a radiation shield; an adjacentuncoated 40 mesh screen of the same type wire and a /2-inch spacing toan outer catalytically coated 30 mesh screen of 0.010-inch Chromel Cwire. A primarily platinum metal catalytic coating was deposited on thisouter screen in accordance with the teachings of the above referred toUnited States Patent No. 2,720,494.

For one test run, air flow of cubic feet per hour (s.c.f.h.) and an airto methane ratio of 10 was used through the unit to provide a resultinghydrocarbon content reading of 8.0 above the open light-off hole in theouter catalytically activated screen and an average reading of 2.4 overthe rest of the outer catalytic radiant screen. The test readings weretaken with a Mine Safety Applicance Meter, Model #40. The temperature atthe outside of the outer (catalytic) screen was 840 F. at the time oftaking the test readings.

In another test run with the same unit, where air flow rate was 350s.c.f.h. and the air-fuel ratio 10 to 1, the resulting hydrocarboncontent reading was 2.8 above the open light-off hole and an average of0.6 over the surface of the catalytic screen. At this fuel rate, thetemperature at the surface of the outer screen was 1170" F.

In still another test run with the same unit, where the air flow ratewas 240 s.c.f.h. and the air fuel ratio was 12 to 1, the resultinghydrocarbon content reading was 8.4 at the open light-off hole and anaverage of 1.7 over the surface of the catalytic screen. At this fuelrate, the temperature at the surface of the outer screen was 950 F.

From the foregoing examples, as well as from other accompanying testoperations, it is found that the second stage catalytic screen is ofparticular advantage in effecting more complete burning of the fuelstream through the heat generating unit; and, that combustionefficiencies range from about 91% to 98.8% above the light-off openingfor a 850 F. to 1200 F temperature range at the surface of the outerscreen; while, in comparison, the efficiencies range from about 98.5% to99.7% for the same temperature range where the gaseous stream carriesthrough the second stage catalytic surface.

I claim as my invention:

1. A catalytic radiant heating apparatus which comprises in combination,an internal gas pervious non-catalytic first stage combustion element ofat least two directly adjacent substantially unspaced apart uncoatedalloy metal gauze mambers adapted to have a gaseous fuel stream passtherethrough, whereby at least one downstream member serves as a burnerscreen and at least one upstream member serves as a radiant heat shieldon the fuel inlet side, a second stage combustion element of at leastone gas pervious alloy metal gauze with a mesh size in the 20 to 70 meshrange having a noble metal gas oxidizing catalytic surface that isspaced a short distance downstream from the face of the burner screen ofsaid first stage element, and confined housing means having air and fuelsupply inlet means thereto enclosing the upstream face of said radiantheat shield member of said first stage element and in additionencompassing the open spacing between said first and second stagecombustion elements.

2. A semi-catalytic radiant heating apparatus adapted to produce heat inthe infra-red range above 900 F. from a gaseous fuel stream, whichcomprises in combination, an uncoated non-catalytic first stage fuel gascombustion element having at least one alloy metal burner screen and atleast one directly adjacent alloy metal gas ditfusing and heat shieldingscreen, the latter being upstream and substantially unspaced from andcoextensive with said burner screen, a catalytically active second stagecombustion element of at least one gas pervious alloy metal screen witha mesh size in the 20 to 70 mesh range coated with a noble metal gasoxidizing deposition, said second stage element being spaced a shortdistance downstream from said first stage element whereby to provide anunobstructed combustion area therebetween, and confined housing meanshaving air and fuel supply inlet means thereto enclosing the upstreamface of said first stage combustion element and in addition encompassingsaid area between said first and second stage combustion elements.

3. A infra-red heat producing unit adapted to provide high temperatureradiant heating above 900 P. which comprises in combination, anon-catalytic alloy metal gas combustion screen directly adjacent to andsubstantially unspaced from and coextensive with at least onenoncatalytic alloy metal gas distributing-heat shielding screen, ahousing means having air and fuel supply inlet means thereto enclosingthe upstream side of said adjacent screens, at least one alloy metalcatalytic screen with a mesh size in the 20 to 70 mesh range having anoble metal gas oxidizing surface that is spaced a short distancedownstream from the outer surface of said gas combustion screen andproviding an unobstructed free area over the surface of said combustionscreen, whereby there may be first stage non-catalytic oxidation ofgases on said combustion screen and a second stage catalytic combustionon said catalytic screen for the gas flow through the unit, and closuremeans around the periphery of said unobstructed area between said firstand second stage combustion elements.

4. The unit of claim 3 further characterized in that said allow metalgauze members for said combustion elements are of chrome-nickel alloywith mesh sizes in the 20 to 70 mesh range.

5. The unit of claim 3 further characterized in that lighting hole meansis provided therethrough downstream from said first stage combustionelement.

6. A semi-catalytic radiant heating unit for producing high temperatureinfra-red heat above 900 F. from a gaseous fuel stream, which comprisesin combination, a substantially flat uncoated. non-catalytic first stagefuel gas combustion element having at least one alloy metal burnerscreen and at least one alloy metal gas dilfusing and heat shieldingscreen, the latter being positioned directly adjacent the upstream faceandsubstantialIy unspaced from and coextensive with said burner screen,confined housing means having air and fuel supply inlet means theretoencompassing the upstream face of said first stage combustion elementand providing a gas manifold section to the latter, removable meansattaching the periphery of said first stage combustion element toexterior wall portions of said housing, a catalytically active secondstage combustion element having at least one gas pervious alloy metalscreen with a mesh size in the 20 to 70 mesh range coated with a noblemetal gas oxidizing deposition that is removably spaced a short distancefrom the downstream face of said first stage combustion element, andperipheral spacer means around the edges of said first and second stagecombustion elements and said housing providing said short spaceddistance between said elements and a resulting unobstructed area overthe downstream face of said burner screen of said first stage combustionelement.

7. The heating unit of claim 6 further characterized in that both thefirst stage and second stage combustion elements are formed of alloymetal gauze members having mesh sizes in the 20 to 70 mesh range andsaid first stage combustion element is transversely supported by aheavier gauge and wider mesh alloy metal supporting member thattraverses the interior of said confined hous- 8. The heating unit ofclaim 6 further characterized in that said second stage combustionelement is coated with a platinum group metal deposition and lightinghole means is provided through said unit to said unobstructed areadownstream for said first stage combustion element.

9. A semi-catalytic radiant heating unit adapted to produce heat in theinfra-red range above 900 F. from a gaseous fuel stream, whichcomprises, in combination, a tubular form internal non-catalytic firststage combustion element having at least one uncoated alloy metal burnerscreen and at least one alloy metal gas diffusing and heat shieldingscreen, the latter extending directly adjacent to and substantiallyunspaced from and coextensive with the upstream face of said burnerscreen, end closure means between the end portions of said tubular fi ststage combustion element with fuel-air inlet means through at least aportion of said end closure means whereby a fuel-air mixture may beuniformly distributed radially outwardly through said tubular combustionelement, a catalytically active second stage combustion element of atleast one gas pervious alloy metal screen with a mesh size in the 20 to70 mesh range coated with a platinum group metal that is spaced fromsaid first stage combustion element, said second stage element being ofa larger but similar tubular configuration to said first stage elementwhereby there is an unobstructed area of substantially uniform depthbetween said tubular elements permitting non-catalytic flame combustionover the downstream face of said burner screen of said first stage ele-9 l0 ment and a second stage combustion of gases passing radi- 3,147,9609/ 1964 Ruif 158-99 ally outwardly through said second stage catalyticele- 1,956,749 5/1934 Terret 158-96 ment, and a resulting substantiallyuniform infra-red heat 3,129,749 4/ 1964 Honger 158-114 emitted radiallyfrom said unit. 3,155,142 11/1964 Stack 158-99 10. The heating unit ofclaim 9 further characterized 5 3,204,6-83 9/1965 Ruff et al. in thatsaid second stage tubular combustion element is 3,219,025 11/1965Webster 158-99 X provided with a small lighting hole permitting ignitionof FOREIGN PATENTS gases on said interior first stage combustionelement.

References Cited 494,087 10/1938 Great Britain. 1,108,655 9/1955 France.

10 UNITED STATES PATENTS 1,136,829 1/1957 France. 2,658,742 11/1953Suter et al. FREDERICK L. MATTESON, JR., Primary Examiner. 3,291,18712/1966 Haensel 158-116 3 09 3 957 Hardison 5 RAMEY, Assistant Examiner-

